In optical recording processes generally a laser beam is modulated, e.g., pulsed corresponding to a pattern of information, and focused onto the surface of a recording layer of a recording element.
The recording layer absorbs sufficient energy at the wavelength of the laser beam to cause small portions of the layer to burn, evaporate or otherwise be deformed. Generally, there is continuous relative motion between the laser and the layer so that, as the laser is pulsed or modulated, discrete pits or holes of varying sizes are created in the layer. The sizes and spacings of these holes constitute the encoded information.
The resulting recorded information element is generally read back by turning down the power of the writing laser or by using another laser to which the layer is transparent, thereby precluding the reading laser from further physically altering the recorded layer. The reading beam is disposed to follow the same path as the recording beam. When the read beam is significantly absorbed by the recording, an optical density difference is detected between pits and unrecorded areas. When the read beam is transmitted by the layer, light scattering caused by the pits and unrecorded areas are detected as an optical density difference.
This density difference is detected by a photodetector positioned to receive laser radiation reflected from the underlying support (in the case of a reflective support) or positioned to receive laser radiation transmitted through the underlying support where holes have been formed in the recording layer (in the case of a transmissive support). The detected density variations are converted back into electrical signals corresponding to the information recorded.
In one type of known recording element, the recording layer comprises a mixture of a binder and dye. The dye has an absorption maximum at or near the wavelength of the laser beam used to thermally deform the recording layer. Known binders include polymer or plastic materials such as cellulose nitrate, cellulose butyrates, polycarbonates, polystyrenes and various rosin derivatives.
In order to be useful in optical recording elements, the binder must be compatible with the selected dye. That is, the binder must be capable of forming an amorphous mixture with the dye at high dye to binder ratios and the amorphous mixture should exhibit a single thermal transition with no phase separation, such as crystallization of the dye or polymer, after annealing. It is also desirable that the binder have (a) a relatively low melt viscosity to facilitate rapid formation of the laser induced deformations, and (b) a relatively high glass transition temperature (Tg) to retain the shape of the deformations after formation thereof.
None of the above mentioned known binders are completely satisfactory. The most frequently used material, cellulose nitrate, is unstable and under certain conditions explosive. Also, recording layers containing cellulose nitrate tend to lose considerable absorption at temperatures above room temperature. Cellulose nitrate also has limited compatibility with certain classes of useful dyes such as infrared absorbing metal dithiene dyes.